1. Field of the Invention
This invention relates to an optical switching apparatus and an optical transmission apparatus which are applied to, for example, a wavelength-division multiplexing optical transmission system.
2. Description of the Related Art
With optical fiber amplifiers recently put to practical use, information transmission by a wavelength-division multiplexing (WDM) method has drawn attention. Optical signals including a plurality of time-division-multiplexed time slots are multiplexed using different wavelengths, which enables the information transmission capacity to be increased remarkably.
In a conventional information transmission system, a transmission apparatus is provided for each wavelength. The add/drop process of signals is carried out in time slots. Since such an architecture requires as many transmission apparatuses as corresponds to the number of wavelengths to be multiplexed, the size of the system becomes large.
To overcome this problem, an optical transmission apparatus capable of performing the add/drop process of signals in wavelengths is being developed. In this type of apparatus, an optical switch apparatus for switching the path of an optical signal is an important device. Hereinafter, an apparatus for carrying out the add/drop process in time slots is called a transmission apparatus and an apparatus for carrying out the add/drop process in wavelengths is called an optical transmission apparatus to distinguish them.
Many information transmission systems are provided with working channels/sections and protection channels/sections for redundancy to prevent the signal transmission from being cut off due to the occurrence of a failure. This type of system has a so-called self-healing function of changing the normal traffic from the working channels/sections to the protection channels/sections or detouring the normal traffic around the working channels/sections to the protection channels/sections.
The self-healing function is a function related to the process called protection switching. The protection switching includes switching whereby the normal traffic flowing through the working channels/sections is detoured to the protection channels/sections and revertive switching whereby the normal traffic flowing through the protection channels/sections is returned to the working channels/sections.
The transmission apparatus can transmit the same traffic to both the working channels/sections and the protection channels/sections, because the traffic transmitted in the form of optical signals are converted to electric signals in the apparatus. This has an advantageous effect on the simplification of the procedure necessary to effect protection switching.
In contrast, the optical transmission apparatus deals with traffic in the form of optical signals without converting traffic into electric signals. Because of this, the conventional optical transmission apparatus cannot transmit the same traffic to both of the working channels/sections and protection channels/sections. This makes the procedure necessary to effect protection switching complex, which leads to the disadvantages that the state where the information transmission is cut off might last a long time and that the switch completion time might become longer.